1. Filed
An embodiment according to the present invention relates to an optical integrator, illumination optical system, exposure apparatus, and device manufacturing method. More particularly, an embodiment according to the present invention relates to an optical integrator used in an illumination optical system of an exposure apparatus used in manufacturing devices such as semiconductor devices, imaging devices, liquid crystal display devices, and thin film magnetic heads by lithography.
2. Explanation of Related Art
The conventional exposure apparatus used in manufacturing the semiconductor devices and others is configured to project and transfer a circuit pattern formed on a mask (reticle), onto a photosensitive substrate (e.g., a wafer) through a projection optical system. The photosensitive substrate is coated with a resist, and the resist is exposed to light by projection exposure through the projection optical system, thereby forming a resist pattern corresponding to the mask pattern. The resolving power of exposure apparatus is dependent on the wavelength of exposure light and the numerical aperture of the projection optical system. Therefore, in order to improve the resolving power of exposure apparatus, it is necessary to decrease the wavelength of exposure light and increase the numerical aperture of the projection optical system.
In general, it is difficult in terms of optical design to increase the numerical aperture of the projection optical system beyond a certain value, and it is thus necessary to decrease the wavelength of exposure light to a shorter wavelength. Therefore, the technique of EUVL (Extreme UltraViolet Lithography) is drawing attention as a next-generation photolithography method of semiconductor patterning (exposure apparatus). The EUVL exposure apparatus uses the EUV (Extreme UltraViolet) light (radiation) having the wavelength in the range of about 5 to 20 nm. When the EUV light is used as exposure light, there is no available optical material that can transmit the light. For this reason, the EUVL exposure apparatus has to use a reflection type optical integrator, a reflection type mask, and a reflection type (catopric type) projection optical system.
In the general exposure apparatus, as well as the EUVL exposure apparatus, it is desirable to form a uniform and rotationally symmetric light intensity distribution (which will also be referred to hereinafter as “pupil intensity distribution”) on an illumination pupil of an illumination optical system. The applicant of the present application proposed the technology of forming an almost uniform and rotationally symmetric pupil intensity distribution on the illumination pupil, by devising a correspondence relation between a plurality of first optical elements in a first fly's eye optical system and a plurality of second optical elements in a second fly's eye optical system in the reflection type optical integrator (See U.S. Pat. Published Application No. 2007/0273859)
The optical integrator disclosed in U.S. Pat. Published Application No. 2007/0273859 is configured so that the plurality of first optical elements in the first fly's eye optical system and the plurality of second optical elements in the second fly's eye optical system are set in optical correspondence in a nearly random form.